U.S. patent application number 11/926757 was filed with the patent office on 2008-05-01 for apparatus and method for attenuating acoustic waves in pipe walls for clamp-on ultrasonic flow meter.
This patent application is currently assigned to CiDRA CORPORATION. Invention is credited to Timothy J. Bailey, Changjiu Dang, Mark Fernald, Daniel L. Gysling.
Application Number | 20080098818 11/926757 |
Document ID | / |
Family ID | 39328550 |
Filed Date | 2008-05-01 |
United States Patent
Application |
20080098818 |
Kind Code |
A1 |
Fernald; Mark ; et
al. |
May 1, 2008 |
Apparatus and Method for Attenuating Acoustic Waves In Pipe Walls
for Clamp-On Ultrasonic Flow Meter
Abstract
An apparatus is presented for damping an undesired component of
an ultrasonic signal. The apparatus includes a sensor affixed to a
pipe. The sensor includes a transmitter and a receiver. The
transmitted ultrasonic signal includes a structural component
propagating through the pipe and a fluid component propagating
through a flow in the pipe. The receiver receives one of the
transmitted components. The apparatus includes a damping structure.
The damping structure dampens the structural component of the
ultrasonic signal to impede propagation of the structural component
to the receiver. The damping structure includes one of a housing
secured to the pipe to modify ultrasonic vibrational
characteristics thereof, a plurality of film assemblies including a
tunable circuit to attenuate structural vibration of the pipe, and
a plurality of blocks affixed to the pipe to either reflect or
propagates through the blocks, the undesired structural component
of the ultrasonic signal.
Inventors: |
Fernald; Mark; (Enfield,
CT) ; Gysling; Daniel L.; (Glastonbury, CT) ;
Bailey; Timothy J.; (Longmeadow, MA) ; Dang;
Changjiu; (Wallingford, CT) |
Correspondence
Address: |
CIDRA CORPORATION
50 BARNES PARK NORTH
WALINGFORD
CT
06492
US
|
Assignee: |
CiDRA CORPORATION
Walliingford
CT
|
Family ID: |
39328550 |
Appl. No.: |
11/926757 |
Filed: |
October 29, 2007 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
60855585 |
Oct 30, 2006 |
|
|
|
Current U.S.
Class: |
73/622 |
Current CPC
Class: |
G01F 1/7082 20130101;
G01F 1/712 20130101; G01N 2291/0422 20130101; G01N 29/222 20130101;
G01N 2291/02836 20130101; G01N 2291/0421 20130101; G01N 2291/048
20130101 |
Class at
Publication: |
73/622 |
International
Class: |
G01N 29/032 20060101
G01N029/032 |
Claims
1. An apparatus for damping at least one component of an ultrasonic
signal, the apparatus comprising: at least one sensor affixed to an
outer surface of a pipe, the at least one sensor having a
transmitter and a receiver, the transmitter for transmitting the
ultrasonic signal, the transmitted ultrasonic signal including a
structural component propagating through a wall of the pipe and a
fluid component propagating through a process flow in the pipe, the
receiver for receiving at least one of the transmitted components
of the ultrasonic signal; and a damping structure affixed to the
pipe and damping the structural component of the ultrasonic signal
to impede propagation of the structural component to the
receiver.
2. The apparatus of claim 1, wherein the damping structure includes
a housing secured to the pipe to modify ultrasonic vibrational
characteristics of the pipe by increasing a flexural stiffness of
the pipe.
3. The apparatus of claim 2, wherein the housing contacts and
reinforces selective areas of the pipe to provide a diaphragm
including reinforced areas and unreinfroced areas, wherein the
unreinforced areas are disposed about and in proximity to the
transmitter and the receiver.
4. The apparatus of claim 3, wherein a resonant frequency of the
diaphragm coincides with a resonant frequency of a maximum
transmission of the ultrasonic signal.
5. The apparatus of claim 3, wherein a diameter of the diaphragm is
twice a thickness of the wall of the pipe.
6. The apparatus of claim 2, wherein the housing further includes
viscoelastic material to provide multiple impedance changes and
alternate energy dissipation paths to augment damping of the
structural component.
7. The apparatus of claim 7, wherein the housing includes slots for
retaining the viscoelastic material.
8. The apparatus of claim 1, wherein the damping structure includes
a plurality of film assemblies applied to an outer surface of the
pipe, each of the film assemblies includes a substrate and a
selectively tunable circuit, wherein the circuit is tuned to
attenuate structural vibration of the pipe and the structural
component propagating in the wall of the pipe.
9. The apparatus of claim 8, wherein the substrate is comprised of
a piezoelectric film and the tunable circuit is comprised of a RLC
circuit.
10. The apparatus of claim 1, wherein the damping structure
includes a plurality of blocks affixed to the pipe.
11. The apparatus of claim 10, wherein the blocks and the pipe wall
are at a different impedance such that the structural component of
the ultrasonic signal is at least one of reflected back toward the
transmitter and propagated through a dissipation path in the blocks
and away from the receiver.
12. The apparatus of claim 10, wherein the blocks are disposed
axially along the pipe between the transmitter and the
receiver.
13. The apparatus of claim 10, wherein one of the blocks is
disposed between the pipe wall and the transmitter and another of
the blocks is disposed between the pipe wall and the receiver.
14. The apparatus of claim 1, further including: a processor
coupled to the receiver and sampling the received components of the
ultrasonic signal, the processor processing the sampled signal to
determine a parameter of the process flow in the pipe.
15. An apparatus for damping at least one component of an
ultrasonic signal, the apparatus comprising: a plurality of damping
blocks disposed axially along an outer surface of a pipe; a
plurality of sensors, each sensor having a transmitter coupled to
one of the damping blocks and a receiver coupled to one of the
damping blocks, the transmitter for transmitting the ultrasonic
signal, the transmitted ultrasonic signal including a structural
component propagating through a wall of the pipe and a fluid
component propagating through a process flow in the pipe, the
receiver for receiving at least one of the transmitted components
of the ultrasonic signal; and a processor coupled to the receiver
and sampling the received components of the ultrasonic signal, the
processor processing the sampled signal to determine a parameter of
the process flow in the pipe.
Description
[0001] This application claims the benefit of U.S. Provisional
Patent Application Ser. No. 60/855,585, filed Oct. 30, 2006.
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS
[0002] This application is also related to U.S. patent application
Ser. No. 11/881,477, filed Jul. 27, 2007. The disclosures of these
U.S. patent documents are incorporated by reference herein in their
entireties.
TECHNICAL FIELD
[0003] This invention relates to a method and apparatus for
attenuating acoustic waves (or ring around acoustics) propagating
through the walls of a pipe for a clamp-on ultrasonic flow
meter.
BACKGROUND
[0004] Most ultrasonic flow measurements seek to leverage
information contained in fluid borne disturbances of a specific
temporal frequency. The specific frequency often results from
natural frequencies of the drive electronics, the transducer, or
the resonant transmission characteristic of the pipe wall.
[0005] Referring to FIG. 8, one of the primary challenges
associated with clamp-on ultrasonic flow metering is the
interference between the structural borne ultrasonic signal
component 100 and the desired fluid borne ultrasonic signal
component 102. The structural borne component 100 of the ultrasonic
signal is often of the same or similar frequency and essentially
masks the fluid borne component 102 of the ultrasonic signal.
[0006] Standard pipes are fairly effective waveguides for
structural borne acoustics components 100. The ultrasonic pulse
propagates along the wall of a pipe 104 with very little damping
and rings around the circumference numerous times until the
inherent damping in the pipe and the propagation of energy axially
away from the initial excitation eventually dissipates the
structural borne ultrasonic waves.
SUMMARY OF THE INVENTION
[0007] In one aspect of the present invention, an apparatus is
presented for damping at least one component of an ultrasonic
signal. The apparatus includes at least one sensor affixed to an
outer surface of a pipe. The sensor includes a transmitter and a
receiver. The transmitter transmits the ultrasonic signal. The
transmitted ultrasonic signal including a structural component that
propagates through a wall of the pipe, and a fluid component that
propagates through a process flow in the pipe. The receiver
receives at least one of the transmitted components of the
ultrasonic signal. The apparatus also includes a damping structure
affixed to the pipe. The damping structure dampens the structural
component of the ultrasonic signal to impede propagation of the
structural component to the receiver.
[0008] In one embodiment, the damping structure includes a housing
secured to the pipe to modify ultrasonic vibrational
characteristics of the pipe by increasing a flexural stiffness of
the pipe. In this regard, the housing contacts and reinforces
selective areas of the pipe to provide a diaphragm including
reinforced areas and unreinforced areas. The unreinforced areas are
disposed about and in proximity to the transmitter and the
receiver. In one embodiment, a resonant frequency of the diaphragm
coincides with a resonant frequency of a maximum transmission of
the ultrasonic signal.
[0009] In another embodiment, the damping structure includes a
plurality of film assemblies applied to an outer surface of the
pipe. Each of the film assemblies includes a substrate and a
selectively tunable circuit. The circuit is tuned to attenuate
structural vibration of the pipe and the structural component
propagating in the wall of the pipe. In one embodiment, the
substrate is comprised of a piezoelectric film and the tunable
circuit is comprised of a RLC circuit.
[0010] In yet another embodiment, the damping structure includes a
plurality of blocks affixed to the pipe. The blocks and the pipe
wall are at different impedances such that the structural component
of the ultrasonic signal is either reflected back toward the
transmitter, or propagates through a dissipation path in the blocks
and away from the receiver. In one embodiment, the blocks are
disposed axially along the pipe between the transmitter and the
receiver. In another embodiment, one of the blocks is disposed
between the pipe wall and the transmitter and another of the blocks
is disposed between the pipe wall and the receiver.
[0011] In still another embodiment, the damping apparatus of the
present invention includes a processor coupled to the receiver. The
processor samples the received components of the ultrasonic signal
and processes the sampled signal to determine a parameter of the
process flow in the pipe.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] Referring now to the drawing wherein like items are numbered
alike in the various Figures.
[0013] FIG. 1 is a block diagram of a flow meter having an array of
ultrasonic sensor units disposed axially along a pipe for measuring
the volumetric flow of the fluid flowing in the pipe, in accordance
with the present invention.
[0014] FIG. 2 is a cross-sectional view of a pipe having a
turbulent flow including coherent structures therein, in accordance
with the present invention.
[0015] FIG. 3 is a block diagram of an alternative embodiment of a
sensing device of a flow meter embodying the present invention
similar to that shown in FIG. 1.
[0016] FIG. 4 is a block diagram of an alternative embodiment of a
sensing device of a flow meter embodying the present invention
similar to that shown in FIG. 1.
[0017] FIG. 5 is a block diagram of an alternative embodiment of a
sensing device of a flow meter embodying the present invention
similar to that shown in FIG. 1.
[0018] FIG. 6 is a block diagram of an alternative embodiment of a
sensing device of a flow meter embodying the present invention
similar to that shown in FIG. 1.
[0019] FIGS. 7A and 7B are a perspective view and a cross-sectional
view along line B-B of FIG. 7A, respectively, illustrating an
embodiment of a structurally significant housing clamped on to a
pipe, in accordance with the present invention.
[0020] FIG. 8 is a cross-sectional view of structurally borne and
fluid borne components propagating through a pipe wall having an
ultrasonic sensor attached thereto.
[0021] FIG. 9A is a cross-sectional view of wrapped and unwrapped
pipe wall having a housing in accordance with the present invention
and one embodiment having no housing.
[0022] FIG. 9B is a table illustrating diaphragm diameter and
ultrasonic frequency as a function of wall thickness.
[0023] FIGS. 10A and 10B are a cross-sectional view and an expanded
view about circle 10B of a structurally significant housing in
accordance with another embodiment of the present invention.
[0024] FIGS. 11A and 11B are a cross-sectional view and a
perspective view of a structurally significant housing in
accordance with another embodiment of the present invention.
[0025] FIG. 12A is an elevational view and a cross-sectional view
of another embodiment of the present invention having piezoelectric
patches for damping structural borne ultrasonic signals in
accordance with present invention.
[0026] FIG. 12B is an elevational view and a cross-sectional view
of the embodiment of FIG. 12A taken along line A-A.
[0027] FIG. 12C is an elevational view and a cross-sectional view
of the embodiment of FIG. 12A.
[0028] FIG. 12D is an elevational view and a cross-sectional view
of the embodiment of FIG. 12A.
[0029] FIG. 13 is a block diagram of a flow logic used in the
apparatus of the present invention.
[0030] FIG. 14 is a k-.omega. plot of data processed from an
apparatus embodying the present invention that illustrates slope of
the convective ridge, and a plot of the optimization function of
the convective ridge.
[0031] FIG. 15 is a block diagram of an apparatus for measuring the
vortical field or other flow characteristics of a process flow
within a pipe, in accordance with the present invention.
[0032] FIG. 16 is a plot of a signal created by a 1 MHz ultrasonic
signal transducer, in accordance with the present invention.
[0033] FIG. 17 is a cross-sectional view of structurally borne and
fluid borne components propagating through a pipe wall having an
ultrasonic sensor attached thereto.
[0034] FIG. 18 is a plot of a received ultrasonic signal along with
an unwanted ring-around signal, in accordance with the present
invention.
[0035] FIG. 19 is a cross-sectional view of structurally borne and
fluid borne components propagating through a pipe wall having an
ultrasonic sensor attached thereto.
[0036] FIG. 20 is a plot showing the phase velocity of supported
circumferential modes with the wall of a pipe, in accordance with
the present invention.
[0037] FIG. 21 is a cross-sectional view of structurally borne and
fluid borne components propagating through a pipe wall having an
ultrasonic sensor attached thereto and having a pair of blocks
affixed to the pipe wall to attenuate the ring-around signal, in
accordance with the present invention.
[0038] FIG. 22 is a plot showing the received signal with and
without ring-around blocks affixed to the pipe wall.
[0039] FIG. 23 is a diagram illustrating the flow of ultrasonic
energy injected into a pipe without ring-around reducing
blocks.
[0040] FIG. 24 is a diagram illustrating the flow of ultrasonic
energy injected into a pipe with ring-around reducing blocks.
[0041] FIG. 25A is a cross-sectional view of a pipe having an
ultrasound transmitter and ultrasound receiver affixed thereto and
illustrating structurally borne and fluid borne components of an
ultrasound signal propagating through the pipe wall and within a
process flow in the pipe.
[0042] FIG. 25B is a cross-sectional view of a pipe having a pair
of blocks affixed to the pipe, an ultrasound transmitter and
ultrasound receiver attached to the blocks and illustrating
structurally borne and fluid borne components of an ultrasound
signal propagating through a pipe wall and within a process flow in
the pipe.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
[0043] The present invention discloses apparatus' and methods for
reducing the impact of structural borne noise, an unintended
by-product of launching a fluid borne ultrasonic interrogation
signal, during the operation of clamp-on flow ultrasonic flow
meters, as described in commonly owned, U.S. patent application
Ser. No. 10/756,977, filed Jan. 13, 2004, which is incorporated
herein by reference.
[0044] FIGS. 1 and 2 illustrate an ultrasonic clamp-on flow meter
110, as described in U.S. patent application Ser. No. 10/756,977,
wherein the ultrasonic flow meter 110 includes an array of
ultrasonic sensors 112 (e.g., a sensing device 112) having a
plurality of ultrasonic sensors 114, 116, 118 and 120 disposed
axially along the length of a pipe 104. Each ultrasonic sensor 114,
116, 118 and 120 comprises a transmitter (TX) 122 and a receiver
(RX) 124 pair. The transmitter 122 provides an ultrasonic signal to
the corresponding receiver 124, wherein the ultrasonic signal is
orthogonal to a direction of the flow of a fluid 126. While this
embodiment of the present clamp-on ultrasonic meter 110 is
described, one will appreciate that the present invention is
applicable to the other embodiments, such as that described and
taught in U.S. patent application Ser. No. 10/756,977, including
embodiments in non-orthogonal ultrasonic signals, pitch and catch
configurations, pulse echo configurations, and combined
transmitter/receiver ultrasonic sensors, as shown in FIGS. 3-6.
[0045] For example, while each of the ultrasonic sensor units 114,
116, 118 and 120 comprises a pair of ultrasonic sensors
(transmitter and receiver) 122, 124 that are diametrically-opposed
to provide through transmission, the present invention contemplates
that one of the ultrasonic sensors 122, 124 of each sensor unit
114, 116, 118 and 120 may be offset axially along the pipe 104 such
that the ultrasonic signal from the transmitter sensor 122 has an
axial component in its propagation direction, as shown in FIG.
3.
[0046] As shown in FIG. 4, the present invention also contemplates
that the sensor units 114, 116, 118 and 120 of the sensing device
112 may be configured in a pulse/echo configuration. In this
embodiment, each sensing unit 114, 116, 118 and 120 comprises one
ultrasonic sensor that transmits an ultrasonic signal through the
pipe wall and fluid 126 substantially orthogonal to the direction
of the fluid flow and receives a reflection of the ultrasonic
signal reflected back from the wall of the pipe to the ultrasonic
sensor.
[0047] Referring to FIG. 5, the sensing device 112 may be
configured to function in a pitch and catch configuration. In this
embodiment, each sensor unit 114, 116, 118 and 120 comprises a pair
of ultrasonic sensors (transmitter, receiver) 122, 124 disposed
axially along the pipe 104 disposed on a same side of the pipe 104
at a predetermined distance apart. Each transmitter sensor 122
provides an ultrasonic signal at a predetermined angle into the
flow 126. The ultrasonic signal propagates through the fluid 126
and reflects off of an inner surface of the pipe 104 which reflects
the ultrasonic signal back through the fluid 126 to a receiver
sensor 124 in the respective pair of sensors 122, 124.
[0048] FIG. 6 shows another pitch and catch configuration for the
sensing device 112 contemplated by the present invention. This
configuration is similar to that shown in FIG. 5 except that the
sensors disposed between the end sensors function as both a
transmitter and a receiver. This pitch and catch configuration
reduces the number of sensors needed to operate.
[0049] Referring back to FIG. 1, the signals S.sub.1(t)-S.sub.N(t)
received from each ultrasonic sensor 114, 116, 118 and 120 are
processed by an ultrasonic signal processor 128 and a signal
processor 130 (having an array processor 131) for determining the
velocity of the fluid flow and/or volumetric flow rate. The signal
processor 130 includes at least one of array processing logic, as
will be described in greater detail hereinafter (See FIGS. 13 and
14); and cross-correlation processing logic, as also will be
described in greater detail hereinafter (FIG. 15).
[0050] One should appreciate that the present invention is
applicable to at least all the configurations of an ultrasonic flow
meter considered herein (as well as others not described herein),
and will be described in greater detail hereinafter. Specifically,
the present invention teaches complimentary approaches to
attenuating or eliminating the structural borne component 100 of
the ultrasonic signal 102 (FIG. 8). For example, one embodiment
comprises a structurally significant housing, a second embodiment
including piezoelectric films applied to the outer surface of the
pipe 104, and a third embodiment includes ring-around blocks
affixed to the outer surface of the pipe 104, each embodiment
directed to, as described herein, dampening the structural borne
ultrasonic component.
[0051] A first embodiment, as shown in FIGS. 7A and 7B, involves
the use of a structurally significant housing 132 to clamp-on to
the outside of the process piping 104. The housing 132 is
structurally significant in terms of mass and stiffness as compared
to the pipe 104 itself and once the clamp-on ultrasonic meter 110
(see FIG. 1) (including the housing 132) is mounted to the pipe
104, the housing 132 and pipe 104 wall essentially form a single
structural body at the ultrasonic excitation frequencies of
interest. By clamping the structurally significant housing 132 to
the pipe 104 with sufficient force, possibly with the addition of
epoxy, the ultrasonic vibrational characteristics of the pipe 104
is effectively modified.
[0052] More specifically, the structurally significant housing 132
essentially modifies the structural properties of the entire
structural path (or substantially the entire path) between the
transmitting and the receiving ultrasonic transducers 122, 124. The
structurally significant housing 132 contacts and reinforces all
areas of the pipe 104 except for the immediate area of the
transmitting and receiving transducers 122, 124 (defined below as
area 103). Given that the flexural stiffness of a plate scales with
the cube of the thickness of the plate, doubling the effective wall
thickness increases the effective flexural stiffness by a factor of
eight (8). Thus, as one rule of thumb, this invention considers
doubling the flexural stiffness by at least two times as being
"significant" and thus a structural housing 132 of the same
material as the pipe 104 need only result in an about twenty-five
percent (25%) increase in effective pipe 104 wall thickness to be
considered significant. Accordingly, the present invention enhances
the relative ability of the transmitting and receiving sensors 122,
124 to communicate with an ultrasonic signal through the fluid 126
by minimizing adverse effects of the structurally borne component
100 of the ultrasonic signal 102.
[0053] In addition to impeding the propagation of the structural
wave component 100 from the transmitting sensor 122 to the
receiving sensor 124, the design of the structurally significant
housing 132 can be optimized to increase the transmission of fluid
borne ultrasonic wave component 102. Referring to FIG. 9A, with the
structurally significant housing 132 in place, an unreinforced area
103 of the pipe 104 wall in proximity to the sensors 122, 124
effectively appears as a clamped "diaphragm" in that the area 103
(e.g., having a diameter "a") about the sensors 122, 124 is more
flexible than an area 105 reinforced by the housing 132.
[0054] Blevins, Formulas for Natural Frequency and Mode Shapes,
(which is incorporated herein by reference) provides formulas for
the natural frequency of a clamped diaphragm. For example, for a
clamped diaphragm having a diameter, a, and thickness, h, for a
material of modulus, E, Poisson ratio, v, and a mass per unit area,
g, the natural frequency may be given by,
f ij = .lamda. ij 2 2 .pi. a 2 [ E h 3 12 g ( 1 - v 2 ) ]
##EQU00001##
where f.sub.ij is tabulated.
[0055] This formulation neglects the real world stiffening effect
of the curvature of the pipe 104 wall in the unreinforced area 103
and thus will likely under predict the natural frequency for a
given geometry. However, recognizing this limitation, initial
calculations show that for a pipe 104 having a wall of about 0.3
inch, and unreinforced sensor areas 103 of roughly about 0.75 inch
in diameters, a flat plate circular disk has resonant frequencies
on the order of 10,000 Hz to 500,000 Hz, which is within the range
of ultrasonic transducers. Thus, tuning the natural frequency of
the diaphragm system 103 that is formed using a structurally
significant housing 132 with the primary transmission frequency of
the ultrasonic sensors 114, 116, 118 and 120--created by either
driving the transducer at a specific frequency, or pulsing the
transducer, is both practical and feasible with commonly available
ultrasonic transducers and the design proposed herein.
[0056] The standard, unreinforced pipe does demonstrate frequency
selectivity with respect to normal incidence ultrasonic waves. The
transmission of normal incident ultrasonic waves 102 (FIG. 8) is
maximized at frequencies that correspond to the wavelength of
compression waves in the pipe 104 wall being an integral number of
halfwave lengths,
.lamda. = 2 t n or f = n c 2 t , ##EQU00002##
Thus, for a 0.3 inch thick steel pipe, maximum transmission occurs
at 340 KHz, 680 KHz, 1020 KHz, etc.
[0057] The effect of the structurally significant housing 132 would
be maximized if the resonant frequency of the diaphragm system
(e.g., areas 103 and 105) designed above coincided with one of the
frequency of maximum transmission.
[0058] The design task of aligning the two resonant frequencies
becomes one of selecting the diameter of the "diaphragm" (e.g.,
areas 103 versus 105) such that the natural frequency of the
"diaphragm" lines up with the frequency of maximum transmission.
Inspection of the above equations shows that this condition is
essentially met for "diaphragms" with radii equal to the thickness
of the pipe 104 wall.
[0059] Thus, under the simplified, but still realistic assumptions
discussed herein, one optimal "diaphragm" diameter (diameter "a" of
area 103) may be equal to two (2) times the thickness of the wall
of the pipe 104. These values are tabulated in Table 1, shown in
FIG. 9B. Note that as the pipe 104 wall gets thicker, the optimal
diaphragm diameter "a" increases. Given the size of conventional
transducers, this effect may be better leveraged for thick wall
pipes, such as those used in high-pressure oil and gas wells.
[0060] Referring to FIGS. 10A and 10B, an additional embodiment of
a structurally significant housing 200 is shown, wherein the
presence of the structurally significant housing 200 provides
multiple impedance changes, alternate energy dissipation paths, and
augmented damping to reduce the level of structural borne noise 100
present to interfere with the fluid borne signal 102 required to
make a flow measurement. Specifically, the structurally significant
housing 200 includes viscoelastic damping material 202 introduced
into slots 204 in the housing 200. For structural waves 100
propagating through the housing, the design of the slots 204
provide for shearing of the viscoelastic material 202, effectively
augmenting the damping of the structural wave 100.
[0061] Referring to FIGS. 11A and 11B, another embodiment of a
structurally significant housing 300 is shown with damping material
202 (e.g., viscoelastic damping material) attached between the
housing 300 and structurally significant plates 302 (e.g., steel
plates) affixed to the housing 300. The structurally significant
housing 300 and the structurally significant plates 302 serve to
constrain the viscoelastic material 202 when deflected, effectively
augmenting the damping of the structural wave 100.
[0062] While the present invention of a structurally significant
housing 132, 200, 300 attenuates the structural borne ultrasonic
signals 100 propagating circumferentially around the pipe 104, one
should appreciate that the housing 132, 200, 300 will also
attenuate or eliminate axially propagating structural borne
ultrasonic signals 100. Further, while the housing 132, 200, 300 is
shown as a single housing comprised of two halves bolted together
to retain the ultrasonic sensors 114, 116, 118 and 120 of the array
of sensors 112, one should appreciate that the present invention
contemplates that the ultrasonic meter may comprise a plurality of
discreet independent structurally significant housings, wherein
each sensor 114, 116, 118 and 120 of the array 112 may be mounted
to the pipe 104 by a respective structurally significant housing
132, 200, 300. It is further contemplated that a housing 132, 200,
300 may also include any number of ultrasonic sensors 114, 116, 118
and 120 less than the total number of the array 112.
[0063] Referring to FIGS. 12A, 12B, 12C and 12D, an additional
approach of attenuating or damping the structural borne ultrasonic
signal or vibration 100 includes the use of piezo films 304 applied
to the outer surface of the pipe 104. Piezo devices 304 bonded to a
vibrating structure and electrically shunted to dissipate charge
generated by deformation are well known to serve as effective
dampening devices for structural vibration, for example, piezo
damping of fan blades and the like. By tuning the electrical
properties of a piezo RLC circuit 306, the circuit 306 can be
optimized to preferentially damp structural vibration of a specific
frequency.
[0064] One objective of the current invention is to bond
piezoelectric materials (e.g. PVDF film) 304 to the pipe 104 wall
along the region of the wall in which the interfering structural
borne ultrasonic vibration 100 (see FIG. 8) would travel. The
circuitry 306 could be broadband in nature or tuned to optimize
attenuation of vibrations at specific frequencies.
[0065] Alternatively to the passive electronic system described
above, the PVDF film 304 could also be used in an active circuit to
preferentially damp out specific structural vibration. One
piezoelectric film 304 contemplated in the present invention is
similar to that shown in U.S. patent application Ser. No.
10/712,833, filed on Nov. 12, 2003, which is incorporated herein by
reference.
[0066] In one configuration envisioned, the PVDF system is applied
to the pipe 104 as a separate sub system of the existing ultrasonic
flow metering system. Typical, piezo transducers are used to launch
and detect ultrasonic signals. The proposed use of piezo dampers
constitute a separate system designed to reduce or eliminate the
structure borne component 100 of the ultrasonic signal,
unintentionally generated as a by-product of generating the fluid
borne component 102, arriving at the ultrasonic detector 124
ideally intended to respond to only fluid borne ultrasonic devices.
An illustration of one embodiment of this concept is shown in FIGS.
12A, 12B, 12C and 12D.
[0067] The compressional wavelength in steel at 1 MHz is
approximately 0.2 inch. Ideally, the spatial extent of the PVDF
patches should target an odd integral number of half wavelengths,
namely about 0.1, 0.3, 0.5 inch and the like.
[0068] Referring back to FIG. 1, the flow logic in the processor
130 may determine the velocity of each sensor in the array of
sensors 114, 116, 118 and 120 using one or both of the following
techniques to determine the convection velocity of vortical
disturbances within the process flow 126 or other characteristics
of the process flow 126 that moves/convects with the process flow
126 by: (1) characterizing the convective ridge of the vortical
disturbances or other characteristics using array processing
techniques that use an array 112 of ultrasonic sensors 114, 116,
118 and 120; and/or (2) cross-correlating unsteady variations in
the ultrasonic signals using ultrasonic sensors 114, 116, 118 and
120. It should be appreciated that while the sensors 114, 116, 118
and 120 have been shown and described, the present invention is not
limited in this regard and the number of sensors can vary. For
example, any number of sensors may be used, such as two (2), three
(3), four (4), . . . to sixteen (16) sensors, without departing
from the scope of the invention.
[0069] Referring to FIG. 13, a block diagram illustrating the flow
logic 308 in the processor 130 of FIG. 1 is shown and is used to
characterize the convective ridge of the unsteady variations of the
ultrasonic signals and determine the flow rates. As shown in FIG.
13, the flow logic 308 includes a data acquisition unit 310 (e.g.,
A/D converter) that converts the analog signals T.sub.1(t) . . .
T.sub.N(t) to respective digital signals and provides the digital
signals T.sub.1(t) . . . T.sub.N(t) to FFT logic 312. The FFT logic
312 calculates the Fourier transform of the digitized time-based
input signals T.sub.1(t) . . . T.sub.N(t) and provides complex
frequency domain (or frequency based) signals
T.sub.1(.omega.),T.sub.2(.omega.),T.sub.3(.omega.), . . .
T.sub.N(.omega.) indicative of the frequency content of the input
signals. It should be appreciated that instead of FFT's, any other
technique for obtaining the frequency domain characteristics of the
signals T.sub.1(t)-T.sub.N(t), may be used. For example, the
cross-spectral density and the power spectral density may be used
to form a frequency domain transfer functions (or frequency
response or ratios) discussed hereinafter.
[0070] One technique of determining the convection velocity of the
coherent structures (e.g., turbulent eddies) 314 within the flow
126 (FIG. 2) is by characterizing a convective ridge of the
resulting unsteady variations using an array 112 of sensors 114,
116, 118 and 120 or other beam forming techniques, similar to that
described in U.S. patent application Ser. No. 09/729,994, filed
Dec. 4, 2000, now U.S. Pat. No. 6,609,069, which is incorporated
herein by reference in its entirety.
[0071] A data accumulator 316 accumulates the frequency signals
T.sub.1(.omega.)-T.sub.N(.omega.) over a sampling interval, and
provides the data to an array processor 318, which performs a
spatial-temporal (two-dimensional) transform of the sensor data,
from the x-t domain to the k-.omega. domain, and then calculates
the power in the k-.omega. plane, as represented by a k-.omega.
plot.
[0072] The array processor 318 may use standard so-called beam
forming, array processing, or adaptive array-processing algorithms,
i.e. algorithms for processing the sensor signals using various
delays and weighing to create suitable phase relationships between
the signals provided by the different sensors, thereby creating
phased antenna array functionality. In other words, the beam
forming or array processing algorithms transform the time domain
signals from the sensor array 112 into their spatial and temporal
frequency components, i.e. into a set of wave numbers given by
k=2.pi./.lamda., where .lamda. is the wavelength of a spectral
component, and corresponding angular frequencies given by
.omega.=2.pi..nu..
[0073] It should be appreciated that the prior art teaches many
algorithms of use in spatially and temporally decomposing a signal
from a phased array of sensors, and the present invention is not
restricted to any particular algorithm. One particular adaptive
array processing algorithm is the Capon method/algorithm. While the
Capon method is described as one method, the present invention
contemplates the use, or combined use, of other adaptive array
processing algorithms, such as MUSIC algorithm. The present
invention also recognizes that such techniques can be used to
determine flow rate, i.e. that the signals caused by a stochastic
parameter convecting with a flow 126 are time stationary and may
have a coherence length long enough so that it is practical to
locate sensors 114, 116, 118 and 120 apart from each other and yet
still be within the coherence length.
[0074] Convective characteristics or parameters have a dispersion
relationship that can be approximated by the straight-line
equation,
k=.omega./u,
where u is the convection velocity (flow velocity).
[0075] Referring to FIG. 14, a k-.omega. plot is a plot of
k-.omega. pairs obtained from a spectral analysis of sensor samples
associated with convective parameters that are portrayed so that
the energy of the disturbance spectrally corresponds to pairings
that might be described as a substantially straight ridge, wherein
the ridge, in turbulent boundary layer theory, is called a
convective ridge.
[0076] To calculate the power in the k-.omega. plane, as
represented by a k-.omega. plot (see FIG. 14) of either of the
signals, the array processor 318 determines the wavelength and so
the (spatial) wavenumber k, and also the (temporal) frequency and
so the angular frequency .omega., of various of the spectral
components of the stochastic parameter. There are numerous
algorithms available in the public domain to perform the
spatial/temporal decomposition of arrays of sensors 114, 116, 118
and 120.
[0077] The present embodiment may use temporal and spatial
filtering to precondition the signals to effectively filter out the
common mode characteristics and other long wavelength (compared to
the sensor spacing) characteristics in the pipe 104 by differencing
adjacent sensors 114, 116, 118 and 120 and retaining a substantial
portion of the stochastic parameter associated with the flow field
and any other short wavelength (compared to the sensor spacing) low
frequency stochastic parameters.
[0078] In the case of suitable coherent structures 314 being
present, the power in the k-.omega. plane shown in the k-.omega.
plot of FIG. 14 shows a convective ridge 320. The convective ridge
320 represents the concentration of a stochastic parameter that
convects with the flow 126 and is a mathematical manifestation of
the relationship between the spatial variations and temporal
variations described above. Such a plot will indicate a tendency
for k-.omega. pairs to appear more or less along a line 320 with
some slope, wherein the slope indicates the flow velocity.
[0079] Once the power in the k-.omega. plane is determined, a
convective ridge identifier 322 (FIG. 13) uses one or another
feature extraction method to determine the location and orientation
(slope) of any convective ridge 320 present in the k-.omega. plane.
In one embodiment, a so-called slant stacking method is used, a
method in which the accumulated frequency of k-.omega. pairs in the
k-.omega. plot along different rays emanating from the origin are
compared, each different ray being associated with a different
trial convection velocity (in that the slope of a ray is assumed to
be the flow velocity or correlated to the flow velocity in a known
way). The convective ridge identifier 322 provides information
about the different trial convection velocities, information
referred to generally as convective ridge information.
[0080] An analyzer 324 examines the convective ridge information
including the convective ridge orientation (slope). Assuming the
straight-line dispersion relation given by k=.omega./u, the
analyzer 324 determines the flow velocity and/or volumetric flow,
which are output as parameters 326. The volumetric flow is
determined by multiplying the cross-sectional area of the inside of
the pipe 104 with the velocity of the process flow 126.
[0081] As previously noted, for turbulent Newtonian fluids, there
is typically not a significant amount of dispersion over a wide
range of wavelength-to-diameter ratios. As a result, the convective
ridge 320 in the k-.omega. plot is substantially straight over a
wide frequency range and, accordingly, there is a wide frequency
range for which the straight-line dispersion relation given by
k=.omega./u provides accurate flow velocity measurements.
[0082] For stratified flows, however, some degree of dispersion
exists such that coherent structures 314 convect at velocities
which depend on their size. As a result of increasing levels of
dispersion, the convective ridge 320 in the k-.omega. plot becomes
increasingly non-linear.
[0083] Another technique for determining convection velocity of the
coherent structures 314 within the flow 126 is by cross-correlating
unsteady pressure variations using an array of unsteady pressure
sensors.
[0084] Referring to FIG. 15, a processor 400 is provided which uses
cross-correlation of unsteady variations of the ultrasonic signals
to determine the flow rates. The processing unit 400 of FIG. 15
determines the convection velocity of the vortical disturbances
within the flow 126 by cross correlating unsteady ultrasonic
variations using an array of ultrasonic sensors 114, 116, 118 and
120, similar to that shown in U.S. Pat. No. 6,889,562, filed Nov.
8, 2001, which is incorporated herein by reference.
[0085] Referring to FIG. 15, the processing unit 400 has two
measurement regions located a distance .DELTA.X apart along the
pipe 104. Each pair of sensors 114, 116 and 118, 120 of each region
act as spatial filters to remove certain acoustic signals from the
unsteady pressure signals, and the distances X.sub.1, X.sub.2 are
determined by the desired filtering characteristic for each spatial
filter, as discussed hereinafter.
[0086] In particular, in the processing unit 400, the ultrasonic
signal T.sub.1(t) is provided to a positive input of a summer 402
and the ultrasonic signal T.sub.2(t) is provided to a negative
input of the summer 402. The output of the summer 402 is provided
to line 404 indicative of the difference between the two ultrasonic
signals T.sub.1, T.sub.2 (e.g., T.sub.1-T.sub.2=T.sub.as1).
[0087] The line 404 is fed to a bandpass filter 406, which passes a
predetermined passband of frequencies and attenuates frequencies
outside the passband. In accordance with the present invention, the
passband of the filter 406 may be set to filter out (or attenuate)
the de portion and the high frequency portion of the input signals
and to pass the frequencies therebetween. Other passbands may be
used in other embodiments, if desired. Bandpass filter 406 provides
a filtered signal T.sub.asf1 on a line 408 to Cross-Correlation
Logic 410, described hereinafter.
[0088] The ultrasonic signal T.sub.3(t) is provided to a positive
input of a summer 412 and the ultrasonic signal T.sub.4(t) is
provided to a negative input of the summer 412. The output of the
summer 412 is provided on a line 414 indicative of the difference
between the two ultrasonic signals T.sub.3, T.sub.4 (e.g.,
T.sub.3-T.sub.4=T.sub.as2). The line 414 is fed to a bandpass
filter 416, similar to the bandpass filter 406 discussed
hereinbefore, which passes frequencies within the passband and
attenuates frequencies outside the passband. The filter 416
provides a filtered signal T.sub.asf2 on a line 418 to the
Cross-Correlation Logic 410. The signs on the summers 402, 412 may
be swapped if desired, provided the signs of both summers are
swapped together. In addition, the ultrasonic signals T.sub.1,
T.sub.2, T.sub.3, T.sub.4 may be scaled prior to presentation to
the summers 402, 412.
[0089] The Cross-Correlation Logic 410 calculates a known time
domain cross-correlation between the signals T.sub.asf1 and
T.sub.asf2 on the lines 408, 418, respectively, and provides an
output signal on a line 420 indicative of the time delay .tau. it
takes for an vortical flow field 314 (or vortex, stochastic, or
vortical structure, field, disturbance or perturbation within the
flow) to propagate from one sensing region to the other sensing
region. Such vortical flow disturbances, as is known, are coherent
dynamic conditions that can occur in the flow which substantially
decay (by a predetermined amount) over a predetermined distance (or
coherence length) and convect (or flow) at or near the average
velocity of the fluid flow. As described above, the vortical flow
field 314 also has a stochastic or vortical pressure disturbance
associated with it. In general, the vortical flow disturbances 314
are distributed throughout the flow, particularly in high shear
regions, such as boundary layers (e.g., along the inner wall of the
pipe 104) and are shown herein as discrete vortical flow fields
314. Because the vortical flow fields (and the associated pressure
disturbance) convect at or near the mean flow velocity, the
propagation time delay .tau. is related to the velocity of the flow
by the distance .DELTA.X between the measurement regions, as
discussed hereinafter.
[0090] Referring to FIG. 15, a spacing signal .DELTA.X on a line
422 indicative of the distance .DELTA.X between the sensing regions
is divided by the time delay signal .tau. on the line 420 by a
divider 424 which provides an output signal on the line 426
indicative of the convection velocity U.sub.c(t) of the saturated
vapor/liquid mixture flowing in the pipe 104, which is related to
(or proportional to or approximately equal to) the average (or
mean) flow velocity U.sub.f(t) of the flow 126, as defined
below:
U.sub.c(t)=.DELTA.X/.tau..varies.U.sub.f(t)
[0091] The present invention uses temporal and spatial filtering to
precondition the ultrasonic signals to effectively filter out the
acoustic disturbances P.sub.acoustic and other long wavelength
(compared to the sensor spacing) disturbances in the pipe 104 at
the two sensing regions and retain a substantial portion of the
ultrasonic signal T.sub.vortical associated with the vortical flow
field 314 and any other short wavelength (compared to the sensor
spacing) low frequency pressure disturbances T.sub.other. In
accordance with the present invention, if the low frequency
pressure disturbances T.sub.other are small, they will not
substantially impair the measurement accuracy of
T.sub.vortical.
[0092] While the cross-correlation was shown using four sensors,
whereby two sensors were summed together to form a sensing region,
the invention contemplates that each sensing region may only be
comprised of one (or more) sensors disposed at an axial location
along the pipe 104.
[0093] As mentioned hereinbefore, the present invention
contemplates that the housing and blocks for attenuating the
structural ultrasonic signals may be used with any configuration of
ultrasonic sensors 114, 116, 118 and 120. Specifically any of the
three classes of flow meters that utilize ultrasonic transducers,
which include transit time ultrasonic flow meters (TTUF), doppler
ultrasonic flow meters (DUF), and cross correlation ultrasonic flow
meters (CCUF).
[0094] CCUF's measure the time required for ultrasonic beams to
transit across a flow path at two, axially displaced locations
along a pipe 104. Within this measurement principle, variations in
transit time are assumed to correlate with properties that convect
with the flow 126, such as vortical structure, inhomogenities in
flow composition, temperature variations to name a few.
[0095] CCUF's utilize high frequency acoustic signals, i.e.
ultrasonics, to measure much lower frequencies, time varying
properties of structures in the flow 126. Like all other cross
correlation based flow meters, the physical disturbances which
cause the transit time variations should retain some level of
coherence over the distance between the two sensors.
[0096] Cross correlation ultrasonic flow meters have been around
since the early 1960's. CCUF's are typically much more robust to
variations in fluid composition than the other ultrasonic-based
flow measurement approaches such as transit time and Doppler based
methods.
[0097] Although CCFU's are operationally more robust than other
ultrasonic interpretation techniques, they suffer from drawbacks
attributed to most cross correlation flow meters, i.e., they are
have slow update rates and relatively inaccurate.
[0098] Transit time, defined as the time required for an ultrasonic
beam to propagate a given distance, can be measured using a
radially aligned ultrasonic transmitter and receiver. For a
homogenous fluid with no transverse velocity components flowing in
an infinitely rigid tube, the transit time may be given by the
following relation:
t=D/A.sub.mix
where t is the transit time, D is the diameter of the pipe 104, and
A.sub.mix is the speed of sound propagating through the fluid
126.
[0099] In such a flow, variation in transit time is analogous to a
variation in sound speed of the fluid. In real fluids however,
there are many mechanisms, which could cause small variations in
transit time which remain spatially coherent for several pipe
diameters. For single phase flows, variations in the transverse
velocity component will cause variations in transit time.
Variations in the thermophysical properties of a fluid such as
temperature or composition will also cause variations. Many of
these effects convect with the flow. Thus, influence of transverse
velocity of the fluid associated with coherent vortical structures
314 on the transit time enables transit time based measurements to
be suitable for cross correlation flow measurement for flows with
uniform composition properties. The combination of sensitivity to
velocity field perturbation and to composition changes make transit
time measurement well suited for both single and multiphase
applications.
[0100] Despite CCUF's functioning over a wide range of flow
composition, standard transit time ultrasonic flow meters (TTUF)
are more widely used. TTUF's tend to require relatively well
behaved fluids (i.e. single phase fluids) and well-defined coupling
between the transducer and the fluid itself. TTUF's rely on
transmitting and receive ultrasonic signals that have some
component of their propagation in line with the flow. While this
requirement does not pose a significant issue for in-line, wetted
transducer TTUF's, it does pose a challenge for clamp-on devices by
introducing the ratio of sound speed in the pipe to the fluid as an
important operating parameter. The influence of this parameter
leads to reliability and accuracy problems with clamp-on
TTUF's.
[0101] CCFU's, utilize ultrasonic transducers to launch and detect
ultrasonic waves propagating normal to the flow path. Refraction of
ultrasonic waves at the pipe/fluid interface is not an issue and
the ratio between sound speed of pipe and the fluid does not direct
effect operability.
[0102] In still another embodiment, each pair of transducers 114,
116, 118 and 120 comprise a single transmitter 122 to emit an
ultrasonic signal through the flow 126 and a receiver, 124 which
receives the respective signal for processing. The time it takes
for the signal to arrive at the receiver transducer 124 for each
pair is calculated and fed to the SONAR algorithms (in the array
processor 131) where the flow rate is calculated. One embodiment
uses a very simplistic signal detection algorithm that looks for a
peak in the reading obtained from the receiver 124. This algorithm
works well when a good signal-to-noise ratio is observed at the
receiver 124, however when bubbles intersect the signal path
between the transmitter 122 and receiver 124 a significant
attenuation can occur, which will severely degrade the received
signal quality. The amount of attenuation will vary depending on
the bubble characteristics such as size and density.
[0103] Referring to FIG. 17, the transmitting ultrasonic transducer
array 122 is periodically pulsed to create the ultrasonic signal
that transmits through the pipe 104 and fluid. Each transducer will
have a fundamental oscillation frequency, which when pulsed will
emit a short ultrasonic burst signal. FIG. 16 shows the signal
created by a 1 MHz ultrasonic transducer when pulsed with a ten
nanosecond (10 ns) width pulse created in the flow meter 110. In
typical applications the receiving ultrasonic transducer 124,
located on the opposite side of a pipe 104, will receive this
signal once it has bisected the pipe 104 however in addition to
this primary through-transmitted signal other unwanted secondary
signals will also be detected. These secondary signals include
portions of the original signal that have been refracted or
reflected along a different path through the pipe 104 than the
preferred direct transmission. Often these secondary signals
possess sufficient strength to still reach the receiver transducer
124 and will interfere with the desired signal. Examples of these
secondary signals include the ring-around signals 600 that travel
within the pipe wall 104, reflected signals 604 that may bounce off
multiple interfaces such as the transducer-pipe interface or the
pipe-liquid interface, or as in the case here where an array of
transducers are used, from an adjacent transducer, as shown in FIG.
17.
[0104] The dominant secondary signal is the `ring-around` signal
600. This is the portion of the ultrasonic signal that travels
around through the wall of the pipe 104 and can still be detected
by the receiving transducer 124. FIG. 18 shows a diagram of this
signal as compared to the through-transmitted signal 602. As shown
in FIG. 19, ultrasonic transmitting and receiving transmitters 122,
124, respectively, are shown attached to the outer surface of a
pipe 104. They are arranged such that the generated ultrasonic
signal will be normal to the direction of the fluid flow and travel
through the center 602 of the liquid within the pipe 104. As
discussed above, as the ultrasonic signal travels through the pipe
104, bubbles 605 (FIG. 17) and other matter within the pipe 104
will scatter and attenuate the signal (e.g., form signals 604)
before it fully traverses the pipe 104 and is detected by the
receiving transducer 124. Also depicted is the `ring-around` signal
600. This signal is created through reflection and diffraction
between the transmitting ultrasonic transducer 122, the pipe wall
104 and the material present inside the pipe 104 due to the large
impedance mismatch between the various materials. As an example,
the impedance of steel such as, for example, in steel piping, is 45
MRayls in contrast to fluid which has an impedance of 1.5 MRayls.
In this case, only a small percentage of the ultrasonic signal is
actually injected into the fluid while the rest is reflected
throughout the overall system. The majority of this excess energy
is present in the pipe 104 wall in the form of shear and
compressional ultrasonic waves 600. These waves will travel
throughout the pipe 104 and will be seen by the receiving
transducer 124 along with any desired signals 602. Coupled with the
fact that the through-transmitted signal 602 can be significantly
attenuated (e.g., signals 604) as it travels through the fluid 126
in the pipe 104, it can be very difficult to distinguish the wanted
signal from all the secondary signals. FIG. 19 shows an example of
a received ultrasonic signal 602 along with an unwanted
`ring-around` signal 600. The arrow indicates the location of the
through-transmitted pulse in relation to the large `ring-around`
signal. Contrast the attenuated ultrasonic signal in FIG. 18 to the
clean ultrasonic signal seen in FIG. 16.
[0105] To increase the system robustness of the ultrasonic flow
meter 110, the amount of the noise signal may be decreased by
mechanically reducing the strength of the secondary ring-around
ultrasonic signals that were able to reach the detectors.
Signal to Noise
[0106] It should be appreciated that the quality of any flow
measurement, independent of the technology, is typically dependent
upon the signal to noise ratio (S/N). Noise, in this case, is
defined as any portion of the measured signal that contains no flow
information. It is desirable to maximize the S/N to obtain optimum
performance. As mentioned, the dominant noise source for the
ultrasonic flow meter 110 was determined to be ring-around noise.
Ring-around noise is defined as the signal (signal 100) seen by the
receiving transducer 124 that has not passed through the fluid 126,
but instead traveled via the pipe 104 wall. This signal 100, 600
contains no flow information and, in certain cases, can corrupt the
measurement of the signal 102, 602 that has passed through the
fluid 126. FIG. 19 shows both the signal path and ring-around
path.
[0107] The ultrasonic flow meter 110 measures the modulation of the
time-of-flight (TOF) measurement orthogonal to the flow direction.
The TOF modulation is due to the vortical disturbances in the beam
path and the flow velocity is determined by correlating these
coherent modulations over the length of the sensor array.
[0108] Under ideal conditions, the ratio of the signal passing
through the fluid 126 to the ring-around noise is high, and/or the
differential TOF between the signals is large, and a flow
measurement can be made. In situations where the straight through
signal is attenuated due to properties of the fluid 124 (air
bubbles, particulates, etc.) the S/N ratio can be substantially
reduced and the flow measurement compromised. In cases where the
signal and noise temporally overlap, and/or in situations where the
ring-around signal is greater than the straight through signal,
advanced signal processing algorithms need to be employed to detect
the signal. In order to reduce the burden placed on the detection
algorithm to detect small signals in the presence of a large
ring-around signal, methods of reducing the amplitude of the
ring-around noise were investigated.
[0109] The properties of the ring-around energy differ depending
upon the wall thickness of the pipe 104, transducer frequency, pipe
surface quality, and transducer size. Generally speaking, higher
levels of ring-around are seen at smaller pipe diameters (e.g.,
about two (2) inches) for a given transducer excitation frequency
due to the tighter curvature of the wall. Ring-around signals can
be generated when energy from the transducer is either directly
coupled into the pipe wall and/or be a result of reflected energy
from the inner pipe/liquid interface. This energy can propagate as
a variety of different waves, such as shear, longitudinal and
surface waves. FIG. 20 shows the phase velocity of supported
circumferential modes within the wall of a schedule 40, 2 inch
steel pipe. It can be seen that at low excitation frequencies, such
as 1 MHz, four modes can be supported in the pipe wall, wherein the
number of modes capable of being supported increases with increased
frequency. The phase velocity of the lower order modes converges to
approximately 3000 meters/sec.
[0110] One approach to eliminate ring-around involves coupling the
energy into a mechanical structure attached to the pipe 104.
Referring to FIG. 21, two blocks 500 (e.g., steel blocks) were
machined with a curvature slightly larger than the radius of a two
inch (2 in) pipe 104. Acoustic coupling gel was applied between the
pipe 104 and the curved face of the blocks 500. The blocks 500 were
then coupled to the pipe 104 which was then filled with water and
the ring-around noise was measured and compared to the straight
through signal. This was accomplished by first measuring and
recording the received signal containing both the ring-around noise
and the straight through signal, followed by a measurement with the
straight through beam blocked. The difference between the
measurements represents the contribution of the ring-around noise.
The results of these tests showed the blocks had little impact on
the attenuation of the acoustic energy propagating in the pipe 104
wall.
[0111] A second test was conducted where the blocks 500 were
affixed to the pipe 104 wall with an adhesive such as, for example,
an epoxy. Comparison of these measurements showed substantial
attenuation of the ring-around energy. FIG. 22 shows the received
signal with and without epoxied ring-around blocks 500. The first
arrival signal without ring-around blocks occurs at approximately
thirty-one micro seconds (31 .mu.secs). This is consistence with
the calculated transit time through steel. The straight through
signal containing the flow information has a transit time of
forty-one micro seconds (41 .mu.sec). Ring-around blocks attenuate
the ring-around noise resulting in an improved signal to noise at
the receiver 124. It should be appreciated that improvements in S/N
of up to twenty decibels (20 dB) were realized with ring-around
blocks.
[0112] It should also be appreciated that while the present
invention contemplates using a block of material 500 (e.g., steel)
attached or engaged to the pipe 104 to attenuate acoustic waves
propagating through the pipe 104 wall, the invention further
contemplates that the blocks 500 may be comprised of a sheet of
material (e.g., steel, tin and lead) that is affixed with epoxy or
otherwise engaged or attached to the pipe 104 wall. The sheet
material may cover a substantial portion of the circumference and
length of the array of sensors 114, 116, 118 and 120. The
attenuation design may comprise of a plurality of respective sheets
for each ultrasonic sensor pair and disposed on both sides of the
pipe 104 between the sensor pair.
[0113] As discussed above and as seen in FIG. 23 and FIG. 24, for
various measurements made on pipes 104 the transit time of an
ultrasonic wave is determined and a related pipe parameter is
derived (e.g. flow velocity). Often the ultrasonic energy is
coupled through a pipe 104 wall and then into the confined fluid
126. The signal of interest is the signal 602 that passes thru the
fluid 126 (or other material contained in the pipe 104). Sometimes
this signal is difficult to see because some of the ultrasonic
energy is unavoidably coupled into the pipe 104 wall and travels
around the circumference of the pipe 104 wall and ends up on top of
the desired signal. This unwanted signal is typically referred to
as ring-round signal 600.
[0114] By attaching blocks 500 with similar impedance to the pipe
104 to the pipe wall the ring-round signal can be reduced. The
blocks 500 reduce the ring-round by basically two methods. First,
for a wave 600 traveling in the pipe wall, the block 500 because of
its thickness, creates a different impedance and the energy is
reflected as signal 610. Second, the energy that is not reflected
travels out into the block 500 as signal 612 and does not continue
around the pipe 104. Note that the blocks 500 should be attached to
the pipe with a solid material because a gel or liquid may not
couple out the shear wave.
[0115] In one embodiment, illustrated in FIGS. 25A and 25B, the
transmitter 122 emits an ultrasonic signal though the pipe 104
resulting in a structural borne signal component 600 that traverses
the pipe wall and a fluid borne signal component 602 that traverses
the fluid 126 (or mixture) in the pipe and are both received by the
receiver 124 (FIG. 25A). As shown in FIG. 25B, a first one of the
blocks 500 is affixed to the pipe 104 between the pipe wall and the
ultrasound transmitter 122, and a second one of the blocks 500 is
affixed to the pipe 104 between the pipe wall and the receiver 124.
As shown in FIG. 25B, the location of the blocks 500 effectively
increases the pipe wall thickness in proximity to the transmitter
122 and the receiver 124 to dissipate and substantially eliminate
the unwanted ring around signal 600 propagating through the pipe
wall. For example, and as is illustrated in FIG. 25B, as the
structural borne signal component 600 exits the block 500 the
component 600 is substantially minimized and dissipates prior to
traversing the circumference of the pipe 104. As such, the
embodiment illustrated in FIG. 25B substantially prevents the
ring-around signal from reaching the receiver 124.
[0116] While the invention has been described with reference to an
exemplary embodiment, it will be understood by those skilled in the
art that various changes may be made and equivalents may be
substituted for elements thereof without departing from the scope
of the invention. In addition, may modifications may be made to
adapt a particular situation or material to the teachings of the
invention without departing from the essential scope thereof.
Therefore, it is intended that the invention not be limited to the
particular embodiment(s) disclosed herein as the best mode
contemplated for carrying out this invention.
* * * * *